Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Financial Assets
Gross Amounts Recognized	$ 33.0	$ 10.1
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	33.0	10.1
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	19.7	2.4
Net Amount	13.2	5.0
Financial Liabilities
Gross Amounts Recognized	30.8	9.4
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	30.8	9.4
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	0.0	4.1
Net Amount	30.7	2.6
Repurchase agreements
Gross Amounts Recognized	636.9	712.4
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	636.9	712.4
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	636.9	712.4
Net Amount	0.0	0.0
Total assets and liabilities
Gross Amounts Recognized	667.7	721.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	667.7	721.8
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	636.9	716.5
Net Amount	30.7	2.6
Interest Rate Swap
Financial Assets
Gross Amounts Recognized	30.8	8.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	30.8	8.8
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	19.7	2.4
Net Amount	11.0	3.7
Financial Liabilities
Gross Amounts Recognized	30.8	8.8
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	30.8	8.8
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	0.0	4.1
Net Amount	30.7	2.0
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	2.2	1.3
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	2.2	1.3
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	2.2	1.3
Financial Liabilities
Gross Amounts Recognized	0.0	0.6
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Net Amounts in the Balance Sheet	0.0	0.6
Financial Instruments	0.0	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0	0.0
Net Amount	$ 0.0	$ 0.6